A study has been made of the biological effect of a small dose of soft x-rays given off by a special water-cooled tube with a window of thin glass, operated at ½ inch spark-gap and 11 milliamperes. Mice exposed for 1 minute show 2 days later in the blood an increase in the number of lymphocytes and in the lymphoid organs an increased number of mitotic figures. There occurs also a marked dilatation of the vessels of the suprarenals, particularly between the cortex and medulla. The latter condition did not appear until after 24 hours and was still present 14 days after the treatment. No change was detected in other organs.

Mice treated in this way showed a high degree of resistance to cancer transplants. The amount of resistance varied with the time of the inoculation after the treatment. The resistance was not increased before 3 days after and was at its highest point 10 days after the treatment.

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